1. Field of the Invention
The present invention relates to a method of correcting data for drawing a photomask. More particularly, the present invention relates to a method of correcting drawing data of a photomask for pattern transfer used in the process of production of a semiconductor or a method of correcting pattern data for preventing pattern deterioration which occurs at the time of wafer transfer.
2. Description of the Related Art
The photomask used in the process of production of a semiconductor device is constructed as a glass substrate on which a light shielding film is formed. The semiconductor is prepared by projecting and exposing the photomask onto a wafer. First, to form the photomask, it is necessary to convert designed computer-assisted design (CAD) data into data for an electron beam (EB) drawing apparatus and faithfully pattern this.
In EB photolithography, however, it has been known that the phenomenon called as the "proximity effect" governs the limit of the resolution and that it has become a serious problem in the preparation of a photomask for performing fine processing and direct drawing of a wafer. The proximity effect is caused by the scattering of electrons in a solid. The effects thereof can be classified into two types according to the shape of the pattern.
Namely, there are the self-proximity effect seen in an isolated fine pattern and the mutual proximity effect seen between adjoining patterns. In the self-proximity effect, the incident electrons are scattered to the outside of the pattern and consequently the accumulated energy within the design dimensions cannot reach a desired value and as a result the final dimensions becomes small or the corner portions are rounded. Further, in the mutual proximity effect, in a gap between the patterns, the accumulated energy reaches the threshold value due to the scattering of electrons from the patterns of the two sides and contact between patterns etc. occur.
As a method of correcting the EB proximity effect, there has already been proposed for a variable shaping type drawing apparatus the method of varying the amount of irradiation for every pattern figure. In this method, as shown in FIG. 1A, an evaluation point is provided at every pattern edge, the accumulated energy found from an exposure intensity distribution (EID) function is calculated at each evaluation point, and the optimum amount of irradiation to be given to the pattern is determined. Note, in this method, in the vicinity of the evaluation points, the amount of irradiation found when the geometrical arrangement of the adjoining patterns are different differs for every evaluation point, consequently the only thing to do is to determine the weighted average value as the amount of irradiation. In this case, sometimes the precision differs for every evaluation point or the intended precision cannot be guaranteed. In order to prevent this, as shown in FIG. 1B, a method of dividing the pattern data so that the amount of irradiation can be controlled for every evaluation point is adopted.
When finely dividing the pattern data existing in all EB data, however, there arises a problem that all of the correction calculation time, data size, and drawing time are increased and a reduction of the mask preparation thruput is induced.
Further, even if the mask can be correctly patterned, pattern deterioration in the wafer, called the light proximity effect, is caused at the time of exposure. This is a phenomenon where the stepper light passing through the opened mask pattern shape diffracts and interferes and consequently the pattern is not correctly resolved on the wafer surface. The light proximity effect includes the self-light proximity effect and the mutual light proximity effect.
The self-light proximity effect means that, in one's own pattern, the stepper light diffracts and, as a result, the final dimensions of the pattern resolved on the wafer become different and the precision of the final dimensions greatly differs in both of the short side and long side in the rectangular pattern. Further, the mutual proximity effect means that, as a result of interference with the stepper light diffracted from the other patterns, the final dimensions on the wafer become small.
For the correction of this light proximity effect, as shown in for example "Automatched deterioration of CAD layout failures through focus: experiment and simulation" (C. A. Spence et al., Proc. of SPIE, Vol. 2197, pp 302-313), there has already been considered the method of correcting the light proximity effect by deforming the pattern figure so that the image is resolved on the wafer in a desired shape by simulating the light intensity on the evaluation points on the pattern edges similar to the proximity effect correction of EB and while shifting the pattern edges.
Also, at the time of this correction, the pattern must be finely divided for improvement of precision, so there arise serious problems of an increase of the correction calculation time, data transfer load due to an increase of the data size, and even the drawing time in a variable shaping type drawing apparatus, a reduction of the thruput, and consequently a delay of they delivery date of products.
Usually, for about a 64 MDRAM, it is sufficient if .+-.0.05 micron is achieved in terms of the dimensional precision on the mask, but a dimensional precision of .+-.0.035 micron is required for about a 256 MDRAM (each dimension is a value on 5X recticle). In general, for the mask data processing for a 0.5 micron rule device such as 16 MDRAM, two hours per layer is set as a reference. It is necessary to suppress the size of the data to about 100 Mbytes in the EB data for drawing a mask. Even for a 64 MDRAM requiring correction of the proximity effect, an equivalent processing ability is required.
The required dimensional precision when forming a pattern on a wafer by using these masks is .+-.0.025 micron in a 0.25 micron rule device such as a 64 MDRAM. In this way, in each data correcting processing for improvement of the precision at the time of formation of a mask corrected for the proximity effect in electron beam drawing and for improvement of the precision at the wafer transfer by correcting the light proximity effect by making corrections to the mask pattern, the calculation must be ended in a practical processing time and with a practical data capacity while satisfying the above precision.
In general, the data for configuring a photomask is comprised by data expressed by just an enormous number of rectangles and trapezoids. The target of the present invention is to prepare corrected data by preliminarily considering the physical phenomenon such as drawing and transfer based on the characteristics of this data.
By this, in the processing of the drawing data of a photomask, it is made possible to prevent the throughput from being reduced by an algorithm for deriving the minimum limit of data division at a high speed for correction of data satisfying the precision required by a photomask and a wafer.